Everything in the Universe is made from energy. It can take many forms: matter, heat, light, electricity and many others. One key fact about energy is that it can change from one form to another.
For example when you walk, the muscles in your legs change the chemical energy in your food into a physical force on your bones. These push your feet against the ground and so produce movement. But movement is also a form of energy. So your muscles have changed chemical energy into movement energy.
Another key fact about energy is that it can never be created or destroyed. The total amount of energy in the Universe today is the same as yesterday and last week and a million years ago.
In most of this story we will be watching how energy changes from one form into another. If these changes did not happen there would be no story, no life, no Universe at all.
Reflections on EnergyEdit
Throughout this story I will be reflecting on why the ideas we meet, such as energy, are important to humans. In the case of energy it is very obvious. We are in the middle of an energy crisis. Modern industrial societies rely to a huge extent on the supply of fossil fuels and as they run out we are constantly seeking new sources of energy.
Looking at the big picture, the food humans eat depends ultimately on plants capturing energy from the Sun in the process called photosynthesis. Even fossil fuels (such as petroleum) are the remains of ancient plants.
Another thing I try to do in this story is to learn lessons from the past to make predictions about the future. Our thoughts about the future tend to come towards the end of the website, but I will add links so, if you want to, you can jump forward to read my thoughts and then use the ‘Back’ button to come back and continue reading. For example you can click here for some thoughts about future sources of energy.
Conservation of EnergyEdit
There’s a fundamental law of physics which says that energy cannot be created or destroyed in this Universe, it can only be changed from one form into another. This is the law of conservation of energy and it has never been found to fail.
The logical conclusion is that all the energy we see in the Universe today must have always been here since the time it was created. And the expansion of the Universe which we saw in the previous chapter tells us that it was smaller in the past.
Energy in PhysicsEdit
In physics, energy (Ancient Greek: ἐνέργεια energeia "activity, operation") is an indirectly observed quantity that is often understood as the ability of a physical system to do work on other physical systems. However, this must be understood as an overly simplified definition, as the laws of thermodynamics demonstrate that not all energy can perform work. Depending on the boundaries of the physical system in question, energy as understood in the above definition may sometimes be better described by concepts such as exergy, emergy and thermodynamic free energy. Therefore, in the words of Richard Feynman, "It is important to realize that in physics today, we have no knowledge what energy is. We do not have a picture that energy comes in little blobs of a definite amount." However, it is clear that energy is always an indispensable prerequisite for performing work, and the concept has great importance in natural science.
Since work is defined as a force acting through a distance (a length of space), energy is always equivalent to the ability to exert pulls or pushes against the basic forces of nature, along a path of a certain length.[dubious – discuss] The total energy contained in an object is identified with its mass, and energy cannot be created or destroyed (thermodynamic free energy, however, can be destroyed). When matter (ordinary material particles) is changed into energy (such as energy of motion, or into radiation), the mass of the system does not change through the transformation process. However, there may be mechanistic limits as to how much of the matter in an object may be changed into other types of energy and thus into work, on other systems. Energy, like mass, is a scalar physical quantity. In the International System of Units (SI), energy is measured in joules, but in many fields other units, such as kilowatt-hours and kilocalories, are customary. All of these units translate to units of work, which is always defined in terms of forces and the distances that the forces act through.
A system can transfer energy to another system by simply transferring matter to it (since matter is equivalent to energy, in accordance with its mass). However, when energy is transferred by means other than matter-transfer, the transfer produces changes in the second system, as a result of work done on it. This work manifests itself as the effect of force(s) applied through distances within the target system. For example, a system can emit energy to another by transferring (radiating) electromagnetic energy, but this creates forces upon the particles that absorb the radiation. Similarly, a system may transfer energy to another by physically impacting it, but in that case the energy of motion in an object, called kinetic energy, results in forces acting over distances (new energy) to appear in another object that is struck. Transfer of thermal energy by heat occurs by both of these mechanisms: heat can be transferred by electromagnetic radiation, or by physical contact in which direct particle-particle impacts transfer kinetic energy.
Energy may be stored in systems without being present as matter, or as kinetic or electromagnetic energy. Stored energy is created whenever a particle has been moved through a field it interacts with (requiring a force to do so), but the energy to accomplish this is stored as a new position of the particles in the field—a configuration that must be "held" or fixed by a different type of force (otherwise, the new configuration would resolve itself by the field pushing or pulling the particle back toward its previous position). This type of energy "stored" by force-fields and particles that have been forced into a new physical configuration in the field by doing work on them by another system, is referred to as potential energy. A simple example of potential energy is the work needed to lift an object in a gravity field, up to a support. Each of the basic forces of nature is associated with a different type of potential energy, and all types of potential energy (like all other types of energy) appears as system mass, whenever present. For example, a compressed spring will be slightly more massive than before it was compressed. Likewise, whenever energy is transferred between systems by any mechanism, an associated mass is transferred with it.
Any form of energy may be transformed into another form. For example, all types of potential energy are converted into kinetic energy when the objects are given freedom to move to different position (as for example, when an object falls off a support). When energy is in a form other than thermal energy, it is theoretically possible to transform it with very high efficiency to any other type of energy, including electricity or production of new particles of matter. (Exactly 100% efficiency is impossible only because of friction and similar losses.) By contrast, there are strict limits to how efficiently thermal energy can be converted into other forms of energy, as described by Carnot's theorem and the second law of thermodynamics.
In all such energy transformation processes, the total energy remains the same, and a transfer of energy from one system to another, results in a loss to compensate for any gain. This principle, the conservation of energy, was first postulated in the early 19th century, and applies to any isolated system. According to Noether's theorem, the conservation of energy is a consequence of the fact that the laws of physics do not change over time.
Although the total energy of a system does not change with time, its value may depend on the frame of reference. For example, a seated passenger in a moving airplane has zero kinetic energy relative to the airplane, but non-zero kinetic energy (and higher total energy) relative to the Earth.